1. Field of the Invention
The present invention relates to a method for connecting an electronic part, especially to a method for connecting an electronic part using an anisotropic conductive adhesive, and to a joined structure.
2. Description of the Related Art
An anisotropic conductive adhesive, in which conductive particles are dispersed in an adhesive, has been conventionally used for a connection of an electronic part.
However, as pitches between terminals of an electronic part have been narrowed in recent years, there are cases where the conductive particles are aggregated between the terminals adjacent each other, causing a short circuit between the terminals.
Especially when a device in which a semiconductor chip is connected to a tape carrier (TAB: Tape Automated Bonding) or a device in which a semiconductor chip is connected to a film carrier (COF: Chip On Film) is connected to a terminal placed at an edge portion of a liquid crystal display panel (LCD panel), a pressure-bonding tool is displaced, and a corner portion (the edge portion) of the LCD panel is pressure-bonded, the conductive particles are accumulated by the edge portion to cause the particle aggregation, causing a short circuit between terminals adjacent each other.
As a technology to reduce occurrences of a short circuit resulted from the aggregation of these conductive particles, there has been proposed to use particles in which an insulating coating is applied to each surface of conductive particles, to reduce the particle diameter of the conductive particles, to reduce the density of the conductive particles, and the like.
The particles applied with the insulating coating have a problem such that a short circuit is caused with the external force enough to destroy the insulating coating at the time when the aggregation is caused.
Moreover, the occurrence of the short circuit due to the particle aggregation cannot completely prevented just by reducing the particle diameter of the conductive particles, and it is also not desirable as the properties (recovery ability and the like) of the conductive particles themselves are reduced.
Furthermore, the proposed method for reducing the particle density to thereby prevent the particle aggregation has a problem such that the capturing of particles between the terminals becomes insufficient, causing a conduction failure.
Moreover, it has been known in the art that an anisotropic conductive adhesive in which insulating particles are added together with the conductive particles is used to completely prevent occurrences of the short circuit due to the particle aggregation (see Japanese Patent Application Laid-Open (JP-A) Nos. 2001-85083, 2005-347273, 2002-75488, 2003-165825, and 08-113654).
However, if the particle diameters of the insulating particles are large, the insulating particles are sandwiched between the terminals of the electronic part before the conductive particles are sandwiched therebetween. If the insulating particles are sandwiched before the conductive particles, the conductive particles are not brought into contact with the terminals, or the pressure applied to the conductive particles are small even in contact with the terminals and the deformation amount thereof becomes small. Therefore, a conduction failure may be occurred between the terminals.
Moreover, after the connection of the electronic part, there may be a case where a number of the captured conductive particles are examined to confirm the connection reliability. When the conductive particles are sandwiched between the terminals, minute uplifts are formed on the back surface of the terminal with a reactive force of the deformation of the conductive particles. Therefore, the number of the conductive particles sandwiched between terminals are confirmed by observing a back surface of the terminal mounted on the surface of the glass substrate from the back surface of the glass substrate of the LCD panel under a differential interference microscope (a differential interferometer) and counting the minute uplifts.
However, if the insulating particles are sandwiched between the terminals, minute uplifts are formed in the same manner as the case where the conductive particles are sandwiched. Here, the minute uplifts formed by sandwiching the insulating particles between the terminal and the minute uplifts formed by sandwiching the conductive particles cannot be distinguished, and thus the number of the captured conductive particles cannot be measured accurately.
Moreover, in the case where the conductive particles are pressed so as to be deformed for the purpose of increasing the contact area between the conductive particles and the terminals, the change of the insulating particles to be sandwiched between the terminals becomes high. Therefore, this problem is especially serious.
Furthermore, the insulating particles are generally constituted of resin particles, and thus the insulating particles tend to absorb the solvent contained in the anisotropic conductive adhesive during or after the production of the anisotropic conductive adhesive. When the insulating particles absorb the solvent, they are swollen and enlarge their particle diameters. Therefore, the problems such as a conduction familiar between the terminals, and measurement of the capturing number become more serious.
In addition to this, if the insulating particles absorb the solvent, the solvent is released from the insulating particles at the time when the anisotropic conductive adhesive is heated, and the solvent is evaporated to thereby form air bubbles (voids) in the anisotropic conductive adhesive.